For twelve years, Chiara De Gregorio and her colleagues awoke before dawn and trudged into the rainforests of Madagascar to observe a critically endangered primate known as the indri, or singing lemur. They battled equipment-soaking downpours, bloodsucking leeches, and uncooperative lemurs, but in the end, the scientists were rewarded with a surprising discovery.

After analyzing hundreds of the primates’ songs, the scientists found that indris sing using a kind of rhythm that only people and birds have been proven to use before. It’s the first time a mammal besides humans has been found to use these rhythms, which are defined as having a set amount of time between notes.  

“Indris are the only lemurs that